1. Field of the Invention
The present invention relates to a continuous process for the production of base compositions for addition-crosslinking or condensation-crosslinking silicone compositions which have improved stability in relation to viscosity increase, SiH degradation, change of reactivity, and also additive degradation after storage.
2. Background Art
Those skilled in the art of the silicone elastomers are aware that silicone compositions comprising filler, for example RTV-2 (Room-Temperature-Vulcanizing 2-component) silicone rubbers, are mainly composed of base compositions. For the production of RTV-2 silicone rubbers, base compositions are mixed homogeneously with a vulcanizing agent, and also with other typical constituents of addition-crosslinking or condensation-crosslinking silicone compositions, which serve to establish certain final properties of the silicone elastomers obtained after the vulcanization process.
Methods based on the prior art for the production of base compositions either use prehydrophobized fillers, which are homogeneously mixed and kneaded together with appropriate organopolysiloxanes, or, in another approach, produce base compositions by using hydrophilic fillers, which are hydrophobized in situ by using suitable treatment agents during what is known as the compounding procedure. In both instances, the incorporation procedure can take place by way of a continuous or batch production process. Features of the batch processes, and in particular here of the in-situ process, are a disadvantageous space-time yield, high energy consumption, and the need to use inert gas because of the risk of formation of explosive mixtures, and these processes moreover have very high capital investment costs and labor costs. Kneaders operating batchwise can produce silicone compositions with high storage stability from base compositions composed of organopolysiloxanes and of prehydrophobized oxidic reinforcing fillers, and the silicone elastomers produced from those base compositions feature a very balanced mechanical property profile.
There are numerous known prior-art technologies for producing silicone compositions by way of a continuous process. The steps in the continuous process can be related either to the continuous production of base compositions or else to the continuous production of finished silicone compositions, the latter being composed of one or more components in most cases.
Silicone compositions which are composed of continuously produced base compositions which use prehydrophobized fillers generally have lower stability when compared with silicone compositions comprising base compositions produced batchwise. The reduced stability of the silicone compositions produced by prior art methods from continuously produced base compositions can become apparent, for example, as an increase in the viscosity of the silicone compositions after storage, which occurs particularly at elevated temperatures, for example during transport of the silicone compositions.
Because the residence time of the organopolysiloxanes and the filler is comparatively low in continuous processes for the production of base compositions, this can also lead to inadequate deactivation of the surface of the filler, with the resultant possibility of undesired reactions in the finished silicone composition. Furthermore, an inadequately deactivated filler surface can lead, in self-adhesive silicone compositions, to undesired reactions of reactive groups at the surface of the filler with additives such as adhesion promoters, with the inevitable result of impaired adhesion properties.
EP 0 807 509 A1 discloses the continuous production of storage-stable addition-crosslinking liquid silicone rubbers by using a specific kneading cascade. A simple devolatizing container is used for devolatization of the base compositions produced by the kneading machine. Although the silicone compositions produced from these base compositions feature good stability with respect to viscosity increase on storage, a substantial disadvantage of the process is that it is only capable of continuous production of storage-stable base compositions in the viscosity range above 500,000 mPas. This process cannot provide stability in the production of organopolysiloxane compositions with viscosity below 500,000 mPas. A further disadvantage of the process described in EP 0 807 509 A1 is that the continuous process can achieve volume flow rates of at most 300 kg/h. At any higher throughput, marked impairment of the stability of the base compositions can be observed.